1. Field of the Invention
The invention relates to radiation detecting arrangements and methods. More specifically, though not essentially, the invention relates to intruder detecting arrangements and methods using passive infra-red (IR) sensors.
2. Prior Art
Intruder detection arrangements employing passive infra-red sensors detect intruders by means of an infra-red radiation sensor having a field of view directed into the area to be protected. If an intruder appears in this field of view, the infra-red radiation which they emit as compared with the immediate surroundings, will be detected by the sensor and an alarm signal will be produced. However, such a system will be rendered ineffective if an object or material which is opaque to IR radiation is accidentally or intentionally placed in the view of the sensor.
A known arrangement for dealing with this problem employs a source of IR radiation associated with the IR sensor. This source directs radiation into the field of view with the intention that it be reflected by any masking material present, the so-reflected radiation being detected by the sensor so as to indicate the presence of the masking material. However, such an arrangement will only detect masking material close to the sensor and only if the masking material is reflective to IR radiation.
Another known arrangement employs a source of IR radiation adjacent to the sensor but outside the window through which the sensor views its field of view, this source directing radiation through the window onto the sensor. Such an arrangement can detect a mask placed directly over the window but cannot detect a mask placed further out into the field of view.
A further known arrangement employs a source of radiation adjacent to the sensor which directs its radiation to a reflector fixed to a wall or other suitable means opposite the sensor. The reflector reflects this radiation back to a special detector, also adjacent to the main sensor. If this special detector receives no radiation from the source, this is indicative of the presence of a mask. However, such an arrangement is limited in that it can only detect masks within the paths from the source to the reflector and from the reflector to the special detector.